Posted By: Vivek Gharote

June 18, 2016
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Overview

Loss due to un-burnt gas (CO): Un-burnt gas loss is mainly due to burning of Carbon to Carbon Monoxide. This can be easily reduced by increasing the supply of Excess Air, which will provide more Oxygen for complete combustion. The measurement of CO at the stack or at the measuring points shown in the figures can guide the operators for correct quantity of air to reduce this loss to zero. Figure shows the loss is high with no excess air but rapidly falls to zero as more air is admitted. Dry flue gas loss As the quantity of excess air is increased, the heat escapes from the stack and the heat goes out of the system. The loss increases in direct proportion to the amount of excess air. Loss due to Combustible in Ash This loss is high when there is little or no excess air because mixing of combustible material and oxygen is poor. As the percentage of excess air is increased, the loss decreases rapidly. However it does not reach zero because the loss depends upon two factors. Firstly the air/fuel mixture which must be correct; secondly upon the fineness of grinding in case of pulverized fuel firing or grate-speed in case of stoker firing. For PF firing, which is the dominant firing method, more finely the coal is grinded, the greater the surface area of coal, hence the combustion is nearly complete and un-burnt loss is at its minimum. In practice though, a stage is reached, where further grinding of coal is not economical. Hence the loss does not reach zero. Generally the coal is grinded to a size of minimum 70% passing through 200 mesh. Combined heat loss If the above three losses are added together then the result is as shown in the figure. The total loss reduces as the excess air is increased; it reaches a minimum and thereafter increases with further addition of excess air. Thus there is a specific quantity of excess air which gives the lowest loss for the combustion of a particular fuel. Expressed otherwise, there is only one value of excess air which will give the maximum efficiency. The value depends upon the quality of fuel used, type of firing, furnace leakages etc. Hence it is utmost important to operate the boiler at optimum level of excess air.

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